1. Field of the Invention
The present invention relates to a technology for decoding an audio signal.
2. Description of the Related Art
Recently, the High-Efficiency Advanced Audio Coding (HE-AAC) method is used for encoding voice, sound, and music. The HE-AAC method is an audio compression method, which is principally used, for example, by the Moving Picture Experts Group phase 2 (MPEG-2), or the Moving Picture Experts Group phase 4 (MPEG-4).
According to encoding by the HE-AAC method, a low-frequency component of an audio signal to be encoded (a signal related to voice, sound, and music etc) is encoded by the Advanced Audio Coding (AAC) method, and a high-frequency component of the audio signal is encoded by the Spectral Band Replication (SBR) method. According to the SBR method, a high-frequency component of an audio signal can be encoded with bit counts fewer than usual by encoding only a portion that cannot be estimated from a low-frequency component of the audio signal. Hereinafter, data encoded by the AAC method is referred to as AAC data, and data encoded by the SBR method is referred to as SBR data.
An example of a decoder for decoding data encoded by the HE-AAC method (HE-AAC data) is explained below. As shown in FIG. 14, a decoder 10 includes a data separating unit 11, an AAC decoding unit 12, an analyzing filter 13, a high-frequency creating unit 14, and a synthesizing filter 15.
When the data separating unit 11 acquires HE-AAC data, the data separating unit 11 separates the acquired HE-AAC data into the AAC data and the SBR data, outputs the AAC data to the AAC decoding unit 12, and outputs the SBR data to the high-frequency creating unit 14.
The AAC decoding unit 12 decodes the AAC data, and outputs the decoded AAC data to the analyzing filter 13 as AAC decoded audio data. The analyzing filter 13 calculates characteristics of time and frequencies related to a low-frequency component of the audio signal based on the AAC decoded audio data acquired from the AAC decoding unit 12, and outputs a calculation result to the synthesizing filter 15 and the high-frequency creating unit 14. Hereinafter, a calculation result output from the analyzing filter 13 is referred to as low-frequency component data.
The high-frequency creating unit 14 creates a high-frequency component of the audio signal based on the SBR data acquired from the data separating unit 11, and the low-frequency component data acquired from the analyzing filter 13. The high-frequency creating unit 14 then outputs the data of the created high-frequency component as a high-frequency component data to the synthesizing filter 15.
The synthesizing filter 15 synthesizes the low-frequency component data acquired from the analyzing filter 13 and the high-frequency component data acquired from the high-frequency creating unit 14, and outputs the synthesized data as HE-AAC output audio data.
Processing performed by the decoder 10 is explained below. The analyzing filter 13 creates low-frequency component data as shown in the left part of FIG. 15. As shown in the right part of FIG. 15, the high-frequency creating unit 14 creates high-frequency component data from the low-frequency component data, and the synthesizing filter 15 synthesizes the low-frequency component data and the high-frequency component data, so that HE-AAC output audio data is created. Thus, the audio signal encoded by the HE-AAC data method is decoded to the HE-AAC output audio data by the decoder 10.
Japanese Patent Application Laid-open No. 2006-126372 discloses an encoding method, according to which when an audio signal is received, and if the audio signal includes an abrupt amplitude change, frequency spectra of the audio signal are divided into a plurality of groups, and bit assignment and quantization are performed on each of the groups.
However, if an audio signal that includes attack sound (a signal including an abrupt amplitude change) is encoded (for example, by the HE-AAC method), and the encoded audio signal is decoded afterward, the above conventional technology cannot properly encode high-frequency component of the audio signal.
A problem in the conventional technology is specifically explained below. As shown in FIG. 16, when encoding an audio signal that includes an abrupt amplitude change within an extremely short time by the SBR method, there is a case where a time region in which the attack sound occurs is extremely short compared with a time region divided by the SBR method due to a characteristic of the SBR method (or the time resolution according to the SBR method is rougher than the time resolution according to the AAC method). The reason for this is because the power of the time region that includes attack sound is evened out, so that attack sound is encoded in a rather slower pace.
The case where the time resolution according to the SBR method is rougher than the time resolution according to the AAC method is explained below. In encoding of an audio signal by the HE-AAC method, encoding is performed by the SBR method at first, and then encoding is performed by the AAC method. In each of the SBR method and the AAC method, encoding is performed by determining whether the audio signal include attack sound, and adjusting the time resolution based on a determination result (if an attack sound is included, the time resolution is set to fine, and if attack sound is not included, the time resolution is set to rough). However, sometimes attack sound is not detected despite that the audio signal includes attack sound. In such case, the time resolution according to the SBR method is rougher than the time resolution according to the AAC method.
In other words, it is strongly required to decode an encoded audio signal properly by compensating a high-frequency component of the encoded audio signal, even if a high-frequency component of the audio signal that includes an attack sound is not properly encoded by the HE-AAC method.